Various types of wastewaters, for example, wastewater generated by mining operations such as drainage from metal mines, and industrial wastewater contain various types of metal ions. Many of metal ions have harmful effects on human bodies or the environment. Therefore, water containing such metal ions must be treated to meet effluent standards prescribed by each country, before being discharged.
Further, recently, some countries and areas are likely to make effluent standards stricter than at present thereby reducing environmental pollution. Accordingly, there are pressing needs for developing techniques for minimizing the concentration of metal ions contained in untreated waste water at low cost.
Various types of wastewaters such as mine drainage or industrial wastewater generally contain ions of metals such as Fe, Zn, Cu, Pb, Cd, and As, and may further contain about 50-3000 mg/L of sulfate ions (SO42−). In view of the above, in recent years, techniques using the action of sulfate-reducing bacteria to remove metal ions from untreated waste water containing sulfate ions have been studied. With such techniques, sulfate ions are reduced by sulfate-reducing bacteria to generate sulfide ions, and by reaction of the sulfide ions with the metal ions, sulfides of the metals are produced and precipitated; thus, the metal ions are removed from the untreated waste water.
In order to sufficiently develop the reaction, the sulfate-reducing bacteria need to be activated. In WO 12/081715 A (PTL 1), the applicant demonstrated that suitable organic-containing materials are required to be selected as organic-containing materials for imparting organic matter that serves as respiratory substrates for the sulfate-reducing bacteria, since a variety of known organic-containing materials include ones that can cause the effects of the sulfate-reducing bacteria and ones that cannot cause the effects of the sulfate-reducing bacteria in practice. Further, the applicant demonstrated that when chaff, which is a biomass resource available at little cost, is used as the organic-containing material, metal ions in untreated waste water can be removed for a long period of time.